1. Field of the Invention
The present invention relates to a substrate processing unit and a substrate processing apparatus, and more particularly to a substrate processing unit and a substrate processing apparatus which are useful for forming a protective film, for example, by electroless plating, on exposed surfaces of embedded interconnects of a conductive material (interconnect material), such as copper or silver, embedded in fine interconnect recesses provided in a surface of a substrate, such as a semiconductor wafer.
The present invention also relates to a substrate holding apparatus and a substrate holding method which can be advantageously used in processing a surface (processing surface) of a substrate with a plating solution or other processing liquid.
2. Description of the Related Art
As a process for forming interconnects in a semiconductor device, a so-called “damascene process”, which comprises embedding a metal (conductive material) into trenches and contact holes, is coming into practical use. According to this process, aluminum, or more recently a metal such as silver or copper, is embedded into trenches and contact holes previously formed in an interlevel dielectric film. Thereafter, an extra metal is removed by performing chemical-mechanical polishing (CMP) so as to flatten a surface.
In a case of interconnects formed by such a process, for example copper interconnects formed by using copper as an interconnect material, embedded interconnects of copper have exposed surfaces after flattening processing. In order to prevent thermal diffusion of such interconnects (copper), or to prevent oxidation of such interconnects (copper) e.g. during forming thereon an insulating film (oxide film) under an oxidizing atmosphere to produce a semiconductor device having a multi-level interconnect structure, it is now under study to selectively cover exposed surfaces of interconnects with a protective film (cap material) composed of a Co alloy, a Ni alloy or the like so as to prevent thermal diffusion and oxidation of the interconnects. Such a Co alloy, a Ni alloy or the like can be produced e.g. by performing electroless plating.
As shown in FIG. 1, for example, trenches 4 as fine interconnect recesses are formed in an insulating film (interlevel dielectric film) 2 of SiO2, low-k material film or the like which has been deposited on a surface of a substrate W, such as a semiconductor wafer. A barrier layer 6 of TaN or the like is formed on an entire surface, and then copper plating, for example, is performed on the surface of the substrate W to fill the trenches 4 with copper and deposit copper film on the surface of the substrate W. Thereafter, CMP (chemical-mechanical polishing) is performed on the surface of the substrate W so as to flatten the surface, thereby forming interconnects 8 composed of a copper film in the insulating film 2. Thereafter, a protective film (cap material) 9 composed of a CoWP alloy film is formed e.g. by electroless plating selectively on surfaces of interconnects (copper) 8 to protect interconnects 8.
A common electroless plating method for selective formation of the protective film (cap material) 9 of the CoWP alloy film on the surfaces of interconnects 8 generally involves the following process steps. First, substrate W such as a semiconductor wafer, which has undergone a CMP process, is immersed in an acid solution e.g. of 0.5M H2SO4 at a solution temperature of e.g. 25° C. for e.g. one minute to remove CMP residue, such as copper, remaining on a surface of an insulating film 2. After cleaning (rinsing) a surface of the substrate W with a cleaning liquid such as pure water, the substrate W is immersed in a mixed solution, e.g. of PdCl2 and H2SO4, for e.g. one minute to adhere Pd as a catalyst to surfaces of interconnects 8, thereby activating exposed surfaces of interconnects 8.
Next, after cleaning (rinsing) the surface of the substrate W with a cleaning liquid such as pure water, the substrate W is immersed in a solution containing e.g. 20 g/L of Na3C6H5O7.2H2O (sodium citrate) at a solution temperature of e.g. 25° C., thereby performing a neutralization treatment of the surfaces of interconnects 8. Then, after cleaning (rinsing) the surface of the substrate W with e.g. pure water, the substrate W is immersed in a CoWP plating solution at a solution temperature of e.g. 80° C. for e.g. 120 seconds, thereby performing selective electroless plating (electroless CoWP cap plating) on activated surfaces of interconnects 8. Thereafter, the surface of the substrate W is cleaned with a cleaning liquid such as ultrapure water. The protective film 9 composed of a CoWP alloy film is thus formed selectively on the surfaces of interconnects 8 to protect interconnects 8.
As described above, when forming a protective film (cap material) composed of a CoWP alloy by electroless plating, a catalyst-application processing for applying a catalyst, for example Pd, to surfaces of interconnects is performed in advance. Further, removal of CMP residue, e.g. copper, remaining on an insulating film, which processing is necessary for preventing a protective film from being formed on the insulating film, is performed usually by using an inorganic acid, such as H2SO4 or HCl. On the other hand, an electroless plating solution is generally an alkaline solution. Accordingly, it is necessary to perform a neutralization step immediately before plating to stabilize a plating process, whereby processes are increased and a number of processing tanks in respective processes is increased. As a result, not only throughput is lowered, but also process control between the processes is complicated. Furthermore, the apparatus has an increased size and occupies a wide installation space in a clean room, leading to an increased cost of the clean room.
This is because though use of a single processing unit to perform different processings with different processing liquids can reduce a space for performing an entire process of substrate processing, and can also reduce energy necessary for substrate transportation, it is difficult to avoid mixing or dilution of different processing liquids when a single processing unit is employed for performing different processings using the different processing liquids.
On the other hand, a dip processing method, which involves immersing a substrate in a processing liquid to bring a surface (processing surface) of the substrate into contact with the processing liquid, has conventionally been employed for performing stable and uniform plating (e.g. electroless plating) of the substrate or stable and uniform pre-plating processing, cleaning or the like of the substrate. A substrate processing unit, adapted for the dip processing method, is generally provided with a substrate holding apparatus for holding a substrate while sealing a peripheral portion of a front surface of the substrate, so that when the substrate, held by the substrate holding apparatus, is immersed in a processing liquid for processing of the substrate, the processing liquid is prevented from intruding into a peripheral portion of the front surface and also into a back surface of the substrate.
A substrate holding apparatus, which employs a so-called vacuum attraction method, has been developed. Such a substrate holding apparatus includes a continuous, ring-shaped attracting seal (annular seal) composed of an elastic material such as a rubber, and presses this attraction seal against a substrate so as to bring an end surface of the attracting seal into tight contact with a peripheral portion of a back surface of the substrate over an entire circumference, and attracts and holds the substrate while sealing the peripheral portion of the back surface of the substrate in a ring shape with the attraction seal by vacuuming an interior of the attraction seal.
It is important for a substrate holding apparatus to be capable of completely releasing a substrate from a head without any load after processing. A conventional method, widely practiced in a substrate holding apparatus adapted for the above-described vacuum attraction method, for example, involves introducing a clean gas, such as N2 gas, into an attracting seal (annular seal) and jetting the gas toward a substrate, thereby releasing the substrate. In some cases, however, a substrate strongly sticking to an attracting seal, such as a rubber, cannot be released only by introduction of clean gas, such as N2 gas. A method is therefore employed to introduce pure water, together with a clean gas, into an attracting seal and jet them toward a substrate simultaneously, thereby securely releasing the substrate even when the substrate is strongly sticking to the attracting seal, which is of a material such as a rubber.
However, when employing a method involving simultaneous jetting of clean gas and pure water, e.g. in a substrate holding apparatus adapted for the vacuum attraction method, two circuits for introductions of a clean gas and of pure water are necessary in addition to a circuit for vacuuming, leading to a complicated circuit construction and an increased size of apparatus.
In performing processing, such as electroless plating, of a substrate while holding the substrate with a substrate holding apparatus, it is desirable to hold the substrate with a weakest possible force and uniformly over an entire surface so as not to cause deformation of the substrate, thereby ensuring accuracy of processing. In order to securely prevent release or fall of a substrate during a series of processings, however, the substrate is attracted or mechanically held by a substrate holding apparatus with a certain degree of holding force which would not cause release or fall of the substrate even when the substrate is rotated generally at a maximum rotational speed, for example, during draining (spin-drying). There are, therefore, cases in which a load is applied locally on a substrate to cause deformation, or a substrate strongly sticks to e.g. an attracting seal whereby release of the substrate becomes difficult.